A displacive structural transformation in the CuO2 planes of YBa2Cu3Ox at the underdoped-overdoped phase separation line

A structural phase transformation in the CuO2 planes of YBa2Cu3Ox has been observed at the onset of the overdoped regime, x = 6.95. We have measured as a function of x the dimpling in the CuO2 planes by EXAFS, and the O(2, 3) in-phase (A(1g)) mode by Raman scattering. The data show for x greater than or equal to 6.95 anomalously large static displacements of the Cu(2) atoms off the O(2, 3) layer and a gap in the distribution of the O(2, 3) in-phase Raman shifts. We conclude the structure of overdoped YBa2Cu3Ox, to be a martensitic form of the optimum doped crystal.